Work-hardened material from stainless steel

ABSTRACT

A stainless steel sheet has a chemical composition consisting of 0.15 mass % or less of C, 1.0 mass % or less of Si, 1.0 mass % or less of Mn, 0.005 mass % or less of S, 10-20 mass % of Cr, 0.5 mass % or less of Ni, 0.001-0.05 mass % of Al, optionally one or more of 0.5-2.0 mass % of Mo, 0.5-3.0 mass % of Cu and 0.05-1.0 mass % of Nb, and the balance being Fe except inevitable impurities. It is strengthened by a work-hardened ferritic structure, wherein Al 2 O 3  and/or Al 2 O 3 .MgO inclusions of 10 μm or less in size are distributed with cleanliness of 0.06% or less. It is useful as lightweight material for parts and structural members of home electric appliances, office automation devices and so on, due to excellent bendability in addition to high yield strength of 700 N/mm 2  or more.

INDUSTRIAL FIELD

The present invention relates to work-hardened stainless steel sheets,having ferritic structures strengthened by work-hardening forimprovement of strength and bendability.

BACKGROUND OF THE INVENTION

There is a demand for lightening members, which are built in homeelectric appliances or office automation devices, e.g. televisions andpersonal computers, especially portable notebook computers. Suchlightening is achieved by reducing thickness of members, but strengthnecessary for use shall be assured regardless of reduction of thickness.In this regard, steel material with 0.2% offset yield strength of atleast 500 N/mm² or Vickers hardness of at least HV200 has been used forthe purpose.

Frames or casings, which are built in home electric appliances or officeautomation devices, are manufactured by pressing or bending cut sheetsto objective profiles. Therefore, metal material for the use shall havegood bendability in addition to mechanical properties.

By the way, provision of naked metal material without necessity ofplating or coating has been earnestly demanded in these days, aiming atenvironmental protection and recyclability. Representative naked metalmaterial with good corrosion-resistant is martensitic stainless steel,e.g. SUS410 or SUS420J2, precipitation-hardening stainless steel, e.g.SUS631, or work-hardening austenitic stainless steel, e.g. SUS304 orSUS301.

Martensitic or precipitation-hardening stainless steel is strengthenedby heat-treatment such as quenching and tempering or aging, after it isformed to an objective profile. However, such heat-treatment is carriedout by a fabricator, so that the fabricator has to bear responsibilityfor a cost of heat-treatment equipment. It is also necessary to pickleor grind a heat-treated steel sheet for removal of oxide scales and toreform the heat-treated steel sheet for elimination of thermaldeformation.

On the other hand, work-hardening austenitic stainless steel has enoughstrength in a state of sheet material with good bendability for afabricator to omit heat-treatment, but is expensive due to addition ofNi at a high ratio. In this regard, there are several proposals onreduction of a Ni content for saving steel costs without degradingadvantages of work-hardening austenitic stainless steel. For instance, aferritic/martensitic dual-phase stainless steel (JP 63-169330 A),wherein its strength and formability are improved by a martensitic phaseand a ferritic phase, respectively. A stainless steel (JP 11-302791 A),wherein its bendability is improved by size- and shape-control of MnSinclusions, which are dispersed in a ferritic/martensitic dual-phase ormartensitic single phase. A stainless steel (JP 2001-262282 A), whereina ferritic structure is work-hardened without heat-treatment.

The ferritic/martensitic dual-phase stainless steel (JP 63-169330 A) hasstrength, which can be raised by an increase of a ratio of a martensiticphase, but its bendability is significantly worsened at an excess ratioof a martensitic phase above 50 mass %.

The stainless steel (JP 11-302791 A) is mainly used as rectangular pipeswith a relatively large bend radius for building constructions. On theother hand, frames, casings or cabinets of home electric appliances areprepared by forming steel sheets to objective profiles with a bendradius remarkably smaller than the rectangular pipes. With the smallbend radius, a dual-phase or martensitic single phase stainless steelsheet is often cracked during formation to a profile of a frame, casingor cabinet, even if MnS is properly controlled in size and shape.

How to control size and shape of MnS is not concretely disclosed in JP11-302791 A. It is well-known that bendability of a steel sheet isworsened by string-shaped MnS, which is expanded along a rollingdirection. MnS is further expanded as an increase of a cold-rollingreduction and finally distributed as fine particles in a steel matrix.As a result, MnS is made harmless due to fine dispersion as for a thinsteel sheet, but still harmful on a relatively thick steel sheet,wherein dispersion of MnS as fine particles can not be expected.Moreover, various alloy designs are necessary in order to ensure properstrength in response to variation of uses, since strength of dual-phaseor martensitic single phase stainless steel without heat-treatment ispredominantly determined by alloying composition.

A cold-rolling method for work-hardening a ferritic structure isadvantageous for improvement of bendability, compared with strengtheningby martensitic transformation. However, JP 2001-262282 A is directed todisc brakes of motorcycles, which are manufactured from stainless steelsheets without bending. A steel sheet, which is manufactured under theproposed conditions, is not proper material for frames, casings orcabinets, since it is often cracked during bending with a small bendradius.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a strengthenedstainless steel sheet, which can be formed to an objective profilewithout cracking even under severe fabricating conditions. Anotherobject of the present invention is to improve formability and strengthof a stainless steel sheet by combination of desulfuring and deoxidizingwith Al for modification of inclusions to fine Al₂O₃ or Al₂O₃.MgOparticles and by cold-rolling for formation of a work-hardened ferriticstructure without necessity of heat-treatment.

The inventive work-hardened stainless steel sheet is characterized byits chemical composition and metallurgical structure.

The stainless steel has a composition consisting of 0.15 mass % or lessof C, 1.0 mass % or less of Si, 1.0 mass % or less of Mn, 0.005 mass %or less of S, 10-20 mass % of Cr, 0.5 mass % or less of Ni, 0.001-0.05mass % of Al and the balance being substantially Fe. The stainless steelmay further contains at least one selected from the group consisting of0.5-2.0 mass % of Mo, 0.5-2.0 mass % of Cu and 0.05-1.0 mass % of Nb.

The stainless steel has a work-hardened ferritic structure, whereinAl₂O₃ and/or Al₂O₃.MgO are distributed as fine particles of 10 μm orless in size with an index of cleanliness of 0.06% or less. Its 0.2%offset yield strength is preferably controlled to a value within a rangeof 500-900 N/mm² by a cold-rolling reduction.

PREFERED EMBODIMENT OF THE INVENTION

MnS, an inclusion harmful on bendability, is relatively soft andexpanded along a rolling direction during cold-rolling, so that it isdispersed as strings in a steel matrix. When such a stainless steelsheet is bent, MnS acts as starting points of cracks due to stressconcentration thereon. Mere desulfurization is insufficient forharmlessness of MnS, but the MnS inclusion is necessarily controlled incomposition, size and shape for inhibition of cracks.

The MnS inclusion changes its composition, size and shape in response toa kind of a deoxidizing agent, which is added to a molten steel in asteel-refining step. When Si is added as a deoxidizing agent forinstance, MnO.SiO₂ and/or MnO.SiO₂.MnS are formed other than MnS.Another deoxidizing agent Ti inhibits formation of string-shapedinclusions, but forms TiN in addition to TiO₂ as a deoxidation product.The reaction products coalesce together into coarse clusters, which willcause surface defects on a stainless steel sheet. Ti-deoxidation is alsoaccompanied with plugging of a tundish nozzle, unless a N content ofmolten steel is specifically reduced.

The inventors have researched and examined various processing conditionsfor inhibition of such inclusions as MnS, MnS, MnO.SiO₂ andMnO.SiO₂.MnS, which worsen bendability of a steel sheet and also degradean external appearance of the steel sheet. In the course of researches,the inventors have discovered that bendability of a steel sheet issurprisingly improved by specified deoxidation with Al, which reformsinclusions to Al₂O₃ or Al₂O₃.MgO type. In fact, a steel sheet, which hasthe structure that Al₂O₃ or Al₂O₃.MgO inclusions of 10 μm in size aredistributed in a steel matrix with an index of cleanliness of 0.06% orless by combination of desulfurization and deoxidation with Al, can beformed to an objective profile with good bendability, as explained inthe below-mentioned examples.

Since a stainless steel sheet is strengthened by cold-working forformation of a work-hardened ferritic phase, 0.2% offset yield strengthsuitable for the purpose is imparted to the stainless steel sheetwithout any special alloy design. Representative cold-working iscold-rolling, and yield strength is adjusted to a value within a rangeof 500-900 N/mm² (i.e. Vickers hardness of 200-300 HV) by control of acold-rolling reduction. By the way, a ferritic stainless steel sheet ina normally annealed state has yield strength of about 250-300 N/mm²(Vickers hardness of about 130-150 HV) too lower than a demand value.

The inventive ferritic stainless steel sheet has the below-mentionedchemical composition with the structure that inclusions are controlledin composition, size and shape.

[Alloy Design]

C: 0.15 Mass % or Less

C is an alloying element for strengthening a steel matrix, but excess Cpromotes precipitation of chromium carbide, resulting in poorcorrosion-resistance. Therefore, an upper limit of a C content isdetermined at 0.15 mass % (preferably 0.08 mass %).

Si: 1.0 Mass % or Less

Si is a ferrite-forming element for strengthening a steel matrix. Butexcess Si above 1.0 mass % unfavorably promotes precipitation of SiO₂ orMnO.SiO₂ inclusions harmful on bendability.

Mn: 1.0 Mass % or Less

Mn, an austenite-forming element, is dispersed as MnO.SiO₂ harmful onbendability in a steel matrix. Therefore, an upper limit of Mn isdetermined at 1.0 mass % (preferably 0.5 mass %).

S: 0.005 Mass % or Less

S dissolves in MnS and MnO.SiO₂ harmful on bendability and forms coarseoxysulfide particles. In order to inhibit harmful effects of S, an upperlimit of S is determined at 0.005 mass % (preferably 0.003 mass %).

Cr: 10-20 Mass %

Cr is an essential element for improvement of corrosion-resistance, anda Cr content of 10 mass % or more is necessary for assurance ofcorrosion-resistance as stainless steel. But excess Cr above 20 mass %degrades toughness of the stainless steel. A Cr content is preferablycontrolled within a range of 11-18 mass %.

Ni is an austenite-forming element. As an increase of a Ni content,stainless steel lowers its Ac₁ temperature and promotes formation of amartensitic phase at a cooling step during annealing. In this regard, aNi content is controlled to 0.5 mass % or less in order to inhibitformation of a martensitic phase.

Al: 0.001-0.05 Mass %

Al is added as a deoxidizing agent. Sufficient deoxidation effect isachieved by controlling an Al content to 0.001 mass % at least. However,excess Al causes massive precipitation of Al₂O₃ particles. The Al₂O₃particles coalesce together into clusters, which unfavorably causesurface defects on a stainless steel sheet. In order to control size ofAl₂O₃ particles to 10 μm or less with cleanliness of 0.06% or less, anupper limit of Al is determined at 0.05 mass %. An Al content ispreferably controlled within a range of 0.003-0.03 mass %.

-   Mo: 0.5-2.0 mass %-   Cu: 0.5-3.0 mass %-   Nb: 0.05-1.0 mass %

Each of Mo, Cu and Nb is an optional element for improvement ofcorrosion-resistance. Effects on corrosion-resistance is noted at 0.5mass % or more of Mo, 0.5 mass % or more of Cu or 0.05 mass % or more ofNb. However, excess Mo above 2.0 mass % worsens cold-workability ofstainless steel due to its solution-hardening effect, excess Cu above3.0 mass % worsens hot-workability and productivity of stainless steel,and excess Nb above 1.0 mass % raises a steel cost without improvementof corrosion-resistance any more.

[A Work-Hardened Ferritic Structure]

Inclusions, which are dispersed in a steel matrix, are reformed to Al₂O₃or Al₂O₃.MgO by desulfurization and deoxidation with Al, and thereformed inclusions are divided to fine particles of 10 μm or less(preferably 5 μm or less) in size by cold-working. The reformation anddividing effectively avoid stress concentration on the inclusions, whichoften act as starting points of cracks. Consequently, a stainless steelsheet can be formed to an objective profile with even a small radius ata bent part, and cracking is remarkably reduced.

Cold-working is advantageous for strengthening a stainless steel sheetin addition to dividing inclusions to fine particles. Namely, a ferriticstainless sheet, which has yield strength of about 250-300 N/mm²(Vickers hardness of about 130-150 HV) in a normally annealed state, isstrengthened by work-hardening. Moreover, a value of yield strength canbe freely adjusted to a value within a range of 500-900 N.mm² (Vickershardness of 200-300 HV) by control of a cold-working reduction, so thatsteel material with strength suitable for a purpose is offered withoutany change of alloy design. In the case where the stainless steel sheetis work-hardened by cold-rolling, a rolling reduction at a finishrolling step is determined within a range of 15-50% (preferably 20-35%)in order to strengthen the stainless steel sheet without degradation ofbendability.

The other features of the present invention will be clearly understoodfrom the following examples.

EXAMPLE 1

Molten stainless steel was deoxidized with Si and adjusted to eachchemical composition shown in Table 1. A sample S-1 is a stainless steelsheet, which was re-crystallized to a single ferritic structure byannealing in succession to hot-rolling and then cold-rolled to thicknessof 1.8 mm with a rolling reduction of 25% for work-hardening theferritic structure. Samples S-2 and S-3 are stainless steel sheets, eachof which was cold-rolled to thickness of 1.8 mm in the same way, held atan elevated temperature in an austenitic/ferritic dual-phase region fora short while and then rendered to a ferritic/martensitic dual-phasestructure by air-cooling. The sample S-2 has a ratio of a martensiticphase greater than the sample S-3. TABLE 1 Chemical Compositions ofStainless Steels (by mass %) C Si Mn S Cr Ni Al S-1 0.068 0.38 0.390.006 12.4 0.4 <0.003 S-2 0.023 0.47 0.85 0.004 11.9 0.09 <0.003 S-30.011 0.24 0.89 0.001 11.7 0.14 <0.003

Test pieces for a tensile test regulated as JIS 13B (JIS Z 2201) weresampled from each stainless steel sheet along two directions, i.e. alongitudinal direction (hereinafter called as “direction-L”) and atransverse direction (hereinafter called as “direction-T”). The testpieces were subjected to a tensile test for measuring yield strength andelongation. Test results are shown in Table 2. It is noted that thesample S-1 had yield strength substantially similar to a value of thesample S-2, which had a martensitic phase at a ratio of 80 vol. %, butits elongation was smaller than the sample S-2. TABLE 2 MetallurgicalStructures and Mechanical Properties Metallurgical Sampling directionY.S. El. Structure of test pieces (N/mm²) (%) S-1 work-hardened L 689 5ferrite T 805 3 S-2 80% martensite + 20% L 708 11 ferrite T 755 11 S-350% martensite + 50% L 591 11 ferrite T 606 12Y.S.: 0.2% offset yield strengthEl.: elongation

Bendability of each stainless steel sheet was evaluated by a V-blockbend method (a V-block bend test by bending angle of 90 degrees underJIS Z2248). Namely, each test piece was bent at a right angle around anaxis in parallel to a rolling direction (hereinafter called as“T-directional bending”) and also around an axis orthogonal to therolling direction (hereinafter called as “L-directional bending”) bypunches with various radii R of top curvatures, and bendability wasrepresented by a minimum radius R, at which the test piece was bentwithout cracking.

It is noted from test results in Table 3 that tendency of crackinitiation in case of T-directional bending was varied among testpieces, although any test piece was not cracked by L-directionalV-bending with even a minimum radius R of 0.1 mm. A minimum radius R was0.6 mm as for crack initiation of the sample S-1 but 1.5 mm as for crackinitiation of the sample S-2, although the samples S-1 and S-2 hadnearly the same yield strength. The comparative result proves that thesample S-1 has bendability better than the samples S-2 and S-3, despiteits elongation smaller than S-2 and S-3. In short, a work-hardenedferritic structure is advantageous in bendability, as compared with amartensitic structure. TABLE 3 Bendability of Each Samples Radius R (mm)of curvature Minimum Bending at a top of a punch radius R direction 0.10.2 0.4 0.6 0.8 1.0 1.5 3.0 (mm) S-1 L ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ <0.1 T X X X X ◯◯ ◯ ◯ 0.8 S-2 L ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ <0.1 T X X X X X X X ◯ 3.0 S-3 L ◯ ◯ ◯ ◯◯ ◯ ◯ ◯ <0.1 T X X X X ◯ ◯ ◯ ◯ 0.8◯ represents crack-free bending, andX represents cracking during bending.

Effects of inclusions on bendability were investigated as follows:

A molten stainless steel was adjusted to the same chemical compositionas the sample S-1, deoxidized with Al and then processed to a sample A-1under the same manufacturing conditions as the above. An Al content ofthe sample A-1, which originated in a deoxidizing agent, was 0.006 mass%, as shown in Table 4.

By EPMA analysis, inclusions of the sample A-1 were identified as amixture of Al₂O₃ and Al₂O₃.MgO, clearly distinguishable from MnO.SiO₂ orMnO.SiO₂.MnS in the sample S-1. Hereinafter, the SiO₂-based inclusion inthe sample S-1 is called as “silicate-type”, while the Al₂O₃-basedinclusion in the sample A-1 is called as “alumina-type”. TABLE 4 Effectsof Deoxidizing Agents on Chemical Compositions Steel Kind C Si Mn S CrNi Al S-1 0.068 0.38 0.39 0.006 12.4 0.40 <0.003 A-1 0.062 0.39 0.270.001 12.6 0.21 0.006

Test pieces JIS 13B for a tensile test were sampled from the samples A-1and S-1 along directions-L and -T, and subjected to a tensile test formeasuring yield strength and elongation. According to test results, thesamples A-1 and S-1 had the same mechanical properties, as shown inTable 5. On the other hand, results of a bending test prove that thesample A-1 had T-directional bendablity apparently superior to thesample S-1, although the samples A-1 and S-1 had nearly the same yieldstrength, as shown in Table 6.

The above results mean that excellent bendability is imparted to astainless steel sheet regardless of strengthening, by combination ofdesulfurization and deoxidation with Al for shape control of inclusionsand by cold-working for formation of a work-hardened ferritic structure.TABLE 5 Effects of Deoxidation with Si or Al on Mechanical PropertiesSampling Deoxidized Composition Metallurgical direction of Y.S. El. byof inclusions structure test pieces (N/mm²) (%) S-1 Si MnO—SiO₂ + work-L 689 5 MnO.SiO₂.MnS hardened T 805 3 ferrite A-1 Al Al₂O₃ + Al₂O₃.MgOwork- L 691 5 hardened T 808 3 ferrite

TABLE 6 Effects of Deoxidation with Si or Al on Bendability Radius (mm)of curvature Minimum Bending at a top of a V-block radius R direction0.1 0.2 0.4 0.6 0.8 1.0 1.5 3.0 (mm) S-1 L ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ <0.1 T X X XX ◯ ◯ ◯ ◯ 0.8 A-1 L ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ <0.1 T ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ <0.1

EXAMPLE 2

Several stainless steels with chemical compositions in Table 7 weremelted in a 30 kg vacuum furnace and deoxidized by addition of an Al orSi deoxidizing agent. TABLE 7 Chemical Compositions of Stainless Steelsand Deoxidizing Steel Deoxidized Alloying components (mass %) Kind By CSi Mn S Cr Ni Al Others A-2 Al 0.05 0.47 0.27 0.001 12.45 0.23 0.004 A-30.01 0.54 0.82 0.001 12.10 0.20 0.008 A-4 0.15 0.62 0.30 0.003 12.400.24 0.004 A-5 0.07 0.54 0.24 0.003 16.45 0.20 0.008 A-6 0.06 0.39 0.450.003 16.75 0.21 0.004 Mo: 0.98 A-7 0.01 0.38 0.24 0.001 16.77 0.250.006 Cu: 1.44 A-8 0.02 0.32 0.95 0.002 18.40 0.20 0.010 Nb: 0.42 A-90.01 0.32 0.21 0.003 17.00 0.11 0.010 Cu: 1.56 Nb: 0.35 B-1 0.06 0.360.29 0.003 12.60 0.23 <0.001   B-2 0.02 0.48 0.78 0.002 16.55 0.10 0.090S-4 Si 0.01 0.40 0.38 0.006 12.4 0.32 <0.001   S-5 0.02 0.47 0.85 0.00211.9 0.09 <0.001   S-6 0.07 0.67 0.02 0.008 16.49 0.24 <0.001  The underlined figures are values out of conditions defined by thepresent invention.

Each stainless steel ingot was forged to a steel plate of 55 mm inthickness and 100 mm in width. The steel plate was ground until itsthickness was reduced to 50 mm. Thereafter, the steel plate washot-rolled to thickness of 5 mm.

Some hot-rolled steel sheets, wherein a martensitic phase was formed byhot-rolling, were annealed for 7 hours at 850° C. and then pickled.Other hot-rolled steel sheets free of a martensitic phase werecontinuously annealed at 1040° C. and then pickled.

As for steel kinds to be strengthened by work-hardening, steel sheetswere cold-rolled to intermediate thickness of 2.3-2.8 mm, continuouslyannealed at 850° C., pickled and then cold-rolled again to finalthickness of 1.8 mm. A total rolling reduction of each steel sheet wascontrolled to a value within a range of 20-35%.

As for steel kinds to be strengthened by martensitic transformation,annealed steel sheets were cold-rolled to intermediate thickness of 3.0mm, re-annealed, pickled and then cold-rolled again to final thicknessof 1.8 mm. The cold-rolled steel sheets were subjected toheat-treatment, i.e. heating for 1 minute at 1000° C. and thenair-cooling, so as to reform its structure to a martensitic single phaseor a ferritic/martensitic dual-phase.

Test pieces were sampled from each stainless steel sheet manufactured asthe above and subjected to observation of a metallurgical structure,inclusions and surface defects. Inclusions were identified by EPMAanalysis, an index of cleanliness was measured by a method regulated inJIS G0555, and a major axis of a biggest inclusion, which was observedin a microscopic view for measurement of cleanliness, was regarded as asize of the inclusion. Other properties, i.e. yield strength, elongationand bendability, were examined by the same way as Example 1.

Results are shown in Table 8. It is noted that test pieces Nos. 1-8,which were strengthened by a work-hardened ferritic structure whereininclusions were shape-controlled by deoxidation with Al, were excellentin bendability regardless of high yield strength of 700 N/mm² or more,since they were bent with a minimum radius R of 0.1 mm or less withoutcracking.

On the other hand, a test piece No. 9, which had high yield strength dueto martensitic transformation, had extremely poor bendability with aminimum bend radius R of 2.5 mm. A test piece No. 10, which was improvedin bendability at the expense of strength by reduction in a proportionof a martensite structure, had a minimum bend radius R of 0.6 mm. Thecomparative results prove that martensitic transformation is effectivefor strengthening a steel sheet but ineffective so much for improvementof bendability.

Even in case of deoxidation with Al, an Al content shall be properlycontrolled for improvement of bendability. Namely, a test piece No. 11with a shortage of Al had poor bendability due to remaining ofsilicate-type inclusions. A test piece No. 12, which was over-deoxidizedwith excess Al of 0.09 mass %, had good bendability but a defectivesurface.

Any of test pieces Nos. 13-15, wherein inclusions were reformed tosilicate-type by deoxidation with Si, was inferior in bendability to thetest pieces Nos. 1-8. TABLE 8 Mechanical Properties and Bendability ofStainless Steel Sheets Inclusions Minimum Steel Deoxidized SizeCleanliness Y.S. El. Bending Surface Note No. Kind by Type (μm) (%)Structure (N/mm²) (%) Radius R(mm) Defects Inventive 1 A-2 Al alumina 30.019 WF 805 3 <0.1 no Examples 2 A-3 alumina 2 0.023 WF 760 4 <0.1 no 3A-4 alumina 4 0.023 WF 823 3 <0.1 no 4 A-5 alumina 3 0.022 WF 810 3 <0.1no 5 A-6 alumina 5 0.020 WF 815 3 <0.1 no 6 A-7 alumina 3 0.018 WF 755 4<0.1 no 7 A-8 alumina 2 0.020 WF 771 3 <0.1 no 8 A-9 alumina 3 0.022 WF781 3 <0.1 no Comparative 9 A-2 Al alumina 3 0.019 F + M 823 9 2.5 noExamples 10 A-3 alumina 2 0.023 F + M 567 14 0.6 no 11 B-1 alumina + 15 0.052 WF 811 3 0.6 no silicate 12 B-2 alumina 20  0.045 WF 768 4 <0.1yes 13 S-4 Si silicate 140  0.081 F + M 578 14 0.8 no 14 S-5 silicate20  0.038 WF 801 3 0.8 no 15 S-6 silicate 210  0.097 WF 815 2 1.0 noThe mark WF is a work-hardened ferritic structure, and the mark F + M isa ferritic/martensitic structure.The underlined figures are values out of conditions defined by thepresent invention.

Test pieces except Nos. 9, 10 and 13 had yield strength of 700 N/mm² ormore due to a work-hardened ferritic structure, which was formed bycold-rolling with a finish rolling reduction of 20-30%. Any of thesetest pieces seems to be poor of ductility from its small elongation of4% or less, but its bendability is actually excellent. The excellentbendability is probably derived from local elongation rather than totalelongation, so that the work-hardened ferritic structure effectivelyimproves local ductility of a bent part at its outside. Moreover, stressconcentration at boundaries between inclusions and a steel matrix isrelaxed by proper shape-control of inclusions. Consequently, cracking isinhibited during forming the inventive stainless steel sheet to anobjective profile

INDUSTRIAL APPLICABILITY OF THE INVENTION

A ferritic stainless steel sheet, proposed by the present invention asmentioned the above, has excellent bendability regardless of high yieldstrength of 700N/mm² or more, since it is strengthened by awork-hardened ferritic structure with inclusions, which haveshape-controlled by deoxidation with Al. The work-hardened stainlesssteel sheet is useful as such without necessity of metal-plating, whichis disadvantageous for environmental protection. Since the stainlesssteel sheet is strengthened by work-hardening, it is formed to anobjective profile at a user aid without necessity of heat-treatment.Furthermore, the stainless steel with a reduced Ni content is useful ascheap material for frames and casings of home electric appliances,office automation devices and so on.

1-3. (canceled)
 4. A work-hardened stainless steel sheet comprising: achemical composition consisting of 0.15 mass % or less of C, 1.0 mass %or less of Si, 1.0 mass % or less of Mn, 0.005 mass % or less of S,10-20 mass % of Cr, 0.5 mass % or less of Ni, 0.001-0.05 mass % of Aland the balance being Fe except inevitable impurities; and awork-hardened ferritic structure, wherein at least one of Al₂O₃ andAl₂O₃.MgO inclusions of 10 μm or less in size are distributed with anindex of cleanliness of 0.06% or less.
 5. The work-hardened stainlesssteel sheet of claim 4, wherein the stainless steel sheet has yieldstrength within a range of 500-900 N/mm².
 6. A work-hardened stainlesssteel sheet comprising: a chemical composition consisting of 0.15 mass %or less of C, 1.0 mass % or less of Si, 1.0 mass % or less of Mn, 0.005mass % or less of S, 10-20 mass % of Cr, 0.5 mass % or less of Ni,0.001-0.05 mass % of Al, and at least one of 0.5-2.0 mass % of Mo,0.5-3.0 mass % of Cu and 0.05-1.0 mass % of Nb, and the balance being Feexcept inevitable impurities; and a work-hardened ferritic structure,wherein at least one of Al₂O₃ and Al₂O₃.MgO inclusions of 10 μm or lessin size are distributed with an index of cleanliness of 0.06% or less.7. The work-hardened stainless steel sheet of claim 6, wherein thestainless steel sheet has yield strength within a range of 500-900N/mm².